The present invention relates to a developing device for a facsimile apparatus, copier, printer or similar image forming apparatus.
It has been customary with an image forming apparatus to use a developing device operable with a single-component type developer, or toner, as distinguished from a two-component type toner made tip of toner and carrier. With this type of developer, or toner as referred to hereinafter, it is possible to reduce the overall size of the developing device and to, in principle, eliminate the need for maintenance. However, such toner suffers from a lack of reliability. Moreover, charging this kind of toner uniformly is so difficult, toner particles charged to polarity opposite to desired polarity are not avoidable. The oppositely charged particles would deposit on and contaminate the background of an image.
The prerequisite with the developing device is that a toner layer be formed on a developing roller or similar developer carrier by uniformly charged toner particles. However, to insure uniform charging, the amount of toner to deposit on the developer carrier should not be excessively great. Furthermore, when the ratio of the liner velocity of the developer carrier to that of a photoconductive element or similar image carrier is excessively great, an excessive scavenging force will act on the toner. At the same time, the toner will be frictionally charged by the image carrier to bring about various problems known in the art.
Whether the toner be charged by friction by a regulating member and first conveying means or by charge injection, the amount of toner to be uniformly charged (i.e., the amount of toner to deposit on the unit area of the first conveying means) should be limited; otherwise, the proportion of oppositely charged toner particles would increase. Therefore, to effect uniform charging, it is not always practicable to deposit the same amount of toner on the first conveying means as needed on the image carrier.
Therefore, the state-of-the-art developing device has to be designed in such a manner as to balance the amount of toner on the developer carrier and the linear velocity ratio of the developer carrier to the image carrier. As a result, the conventional developing device is not always capable of producing attractive images.